1. Field of the Invention
The present invention relates to a system for reproducing moving pictures in a special manner in an encoding system and a decoding system for picture data containing both intra-frame encoded picture data and inter-frame predictively encoded picture data.
2. Description of the Related Art
A system for reproducing picture data stored on a recording medium such as a CD-ROM and displaying the reproduced picture data as moving picture data on a display unit has been commonly used. In addition, a service such as VOD (Video On Demand) for receiving picture data stored in a remote server through a network and reproducing the picture data as moving picture data is also becoming common.
Normally, moving picture data is compressed (encoded) before it is stored or transmitted. As a method for compressing the moving picture data, the MPEG (Moving Picture coding Experts Group) method that has been proposed as an international standard is well known.
In the MPEG method, picture data is composed of three types of data that are I picture data, P picture data, and B picture data. The I picture (Intra-frame picture) data is data that is compressed corresponding to the intra-frame encoding method. One frame of picture data is reproduced with one I picture data. The P picture (Predictive picture) data is data that is compressed corresponding to the inter-frame predictive encoding method based on a moving compensative predictive method with a past frame of picture data. The B picture (Bidirectionally predictive picture) data is data that is compressed corresponding to the inter-frame predictive encoding method based on a moving compensative predictive method with a past frame or a future frame of picture data.
FIG. 1 is a block diagram showing a conventional picture encoding/decoding method. An encoding unit 201 receives bit-map format picture data and encodes (compresses) the picture data. In other words, an encoding unit 202 performs an encoding process for each frame and generates I picture data, P picture data, and B picture data. A buffer memory 203 temporarily stores the encoded picture data, reads the data at a predetermined timing, and sends the data to a packet assembling unit 204. The packet assembling unit 204 assembles the picture data that has been encoded by the encoding apparatus 201 as packets in the format corresponding to the MPEG method, and writes the packets on a storage medium 205.
When picture data is reproduced, a packet disassembling unit 206 sends a bit stream of picture data contained in the packets read from the storage medium 205 to a decoding apparatus 207. The decoding unit 207 temporarily stores the data received from the packet disassembling unit 206 in a buffer memory 208. A decoding unit 209 reads picture data from the buffer memory 208 at a predetermined timing, decodes (expands) the picture data, and writes the decoded data to a frame memory (not shown). The picture data written in the frame memory is reproduced on a display unit.
When the picture data is reproduced, a special reproducing operation such as a high speed reproducing operation or a high speed reverse reproducing operation may be required. This request is issued by, for example, a user. A controller 210 starts or stops the special reproducing process corresponding to such a request. Such a special reproducing operation is performed by decoding only I picture data compressed corresponding to the intra-frame encoding method. There are two decoding methods for decoding only the I picture data.
In the first method, the I picture data is extracted and decoded by the decoding unit 207. In the second method, the I picture data is extracted from the storage medium 205. The extracted I picture data is decoded by the decoding unit 207.
In the first method, when the special reproducing operation is performed, data that is the same as that for the normal reproducing operation is sent from the storage medium 205 to the decoding unit 207. When a three-times-normal speed reproducing operation is performed, data that is the same as that for the normal reproducing operation is sent at a speed three times higher than that for the normal reproducing operation. In other words, even if the special reproducing operation is performed, the I picture data, the P picture data, and the B picture data are sent from the storage medium 205 to the decoding unit 207. However, when the above-described special reproducing operation is performed, since only the I picture data is reproduced, the P picture data and the B picture data are removed, and are not used. Thus, in the first method, since data that is not reproduced is sent, the data transmission rate should be increased. Consequently, the cost of the final apparatus will rise. In addition, since a function for extracting the I picture data from data transmitted at a higher speed than the normal reproducing operation and for decoding the extracted I picture data is required, the cost of the final apparatus will further rise.
As an example of the second method, I picture data is stored on the storage medium 205 along with the normal picture data (packets of which picture data, composed of I picture data, P picture data, and B picture data, are divided into). When the special reproducing operation is performed, the I picture data is read and decoded. In this method, when the special reproducing operation is performed, since only the I picture data is sent to the decoding apparatus 207, the problem of the first method is solved. However, in the second method, an area for storing the I picture data is required along with an area for the normal picture data. Thus, the storage area of the storage medium 205 should be increased.